Breaking this down, several of AWS's core repos like the JS SDK use an allowlist of which contributor ids can run workflow actions in their PRs. The list was a regex, contained several short ids, and wasn't anchored with ^$, so if it allowed user 12345, then any userid containing 12345 could run their own actions on the PR, including one that exfiltrated access tokens. So they spammed GH with user creation requests, got an id that matched, and they were in like Flynn.

Said tokens didn't have admin access, but had enough privileges to invite other users to become full admins. Not sure if they were rotated, but github tokens are usually long-lived, like up to a year. Hey, isn't AWS the one always lecturing us to use temporary credentials? To be fair, AWS did more than just fix the regex, they introduced an "approve workflow run" UI unto the PR process that I think GH is also using now (not sure about that).